Including the same header file in same compilation unit is a problem because it violates a basic rule of C++: One Definition Rule (ODR). A header may get included multiple times because of direct and indirect inclusion.

Include Guard macro idiom is an old idiom, which is also applicable in a C program. It used simple #define to prevent inclusion of a header file multiple times in a compilation unit. Following macros are put at the very beginning and at very end of a header file.

Note:
Programmers often have their include guard macros start with one or more underscores, followed by uppercase letters, even though such identifiers are officially reserved for the implementation of the compiler and the Standard Library, according to the C++ Standard (ISO/IEC 14882:2003).

Some compilers support

#pragma once

as an efficient alternative to include guards. It does not require to open the header file more than once, unlike traditional include guard macro in some compilers. On many modern compilers like GCC4 or MSC++2008 #pragma once will not give better compile time performance as they recognize header guards.